Seriously needs a better title. Alice has just brought home a new puppy! It's adorable at first, but Alice soon starts to notice it behaving strangely. She takes the animal to a vet, or does some research of her own, only to find that her new pet is not a dog at all. The animal's true identity could be anything from an alien lifeform to a Mexican sewer rat. The most common variant involves the owner mistaking it for a dog, but other pet variants are possible. See also I Am Not Weasel, All Animals Are Dogs. If another animal is deliberately masquerading as a dog, or has been disguised as one by a human, it may be Ass in a Lion Skin. When the creature is a sentient lifeform trying to pass as a dog, it's The Dog Is an Alien.
- Played for Laughs in a commercial for Sears Optical: A lady badly in need of glasses lets a racoon into her house, thinking it's her pet cat.
- In Evolution, someone finds a "dog" which is actually one of the violently aggressive aliens. Hilarity Ensues.
- To George of the Jungle, Shep the elephant is his "big, gray, peanut-loving poochie." And Shep seems to believe it too.
- Happens in Adventures in Babysitting, when Brenda has lost her glasses and is stuck in the bus station. She sees what she thinks is a kitten, picks it up... and it turns out to be a jumbo-sized sewer rat. Watch it here, at about 8:05.
- In the original Ghostbusters, this is a Running Gag with Gozer's two minion creatures. Rick Moranis' character thinks they're dogs, one of his party-goers called one a bear, a policeman explained to Venkman that it was a cougar, and near the end Venkman concedes that his date has become a "dog."
- Played completely and horrifyingly straight in The Thing, where the characters bring a malamute home to their base camp and it turns out to be an alien bent on assimilating all life on Earth.
- In Piers Anthony's Race Against Time, one clue that tips off to the protagonist that he's living in a human zoo is that his new "puppy" climbs trees, and his "kitten" has a prehensile tail. Whether they're aliens that only look like house pets, or actual pets modified via genetic engineering, he never does find out.
- In Howard Waldrop's short story The Ugly Chickens, a family has been raising what they think are just very ugly chickens for several hundred years. It turns out that they're actually dodo birds.
- ALF would sometimes be mistaken for a dog, even getting thrown into the pound in one episode.
- The urban myth of "The Mexican Pet" involves a woman taking her "dog" to the vet, only to be told that it's a diseased Mexican sewer rat.
- There's the old joke that ends, "What the hell kind of dog is that?" (After said "dog" bites off the head of a German shepherd.)
"Well, afore I bobbed his tail and painted him yellow, he was an alligator."
- Happens in a StarCraft cutscene: "Looks like you mashed some poor feller's dog, sarge." "It's a zergling, Lester, smaller type of zerg. They shouldn't be out this far unless... oh shit."
- Tales of Symphonia has Noishe, who Lloyd thinks is just an oversized dog. He's actually Protozoan, a very ancient species.
- The Tick mistakes a capybara for a dog and makes a pet of it.
- Inverted with Ren of Ren and Stimpy; most people think he's some sort of rat, though he's really a chihuahua.
- In The Looney Tunes Show, Bugs mistakes the Tasmanian Devil for a dog.
- In the first episode of The Mighty B!, the dog Portia enters in the dog show turns out to be a rat.
- Naturally, this sort of thing happens to Mr. Magoo all the time. One cartoon has him releasing a panther from a circus when he mistakes it for his dog being taken away by the dog catcher. In the feature film 1001 Arabian Nights, Magoo's "dog" Bowser is actually a cat, and a subplot has a ball of enchanted yarn used to make a Flying Carpet takes a liking to Magoo after he mistakes it for a cat and starts to pet it.
- Kim Possible: Kim and Ron go undercover at a dog show, entering Rufus (a naked mole rat) as a contestant. Rufus wins!
- The Goode Family adopt what looks like a diseased dog. When they take it to the vet, they discover that it's actually a rare Malasian marsupial, whose grotesque appearance is perfectly natural.
- On The Grim Adventures of Billy & Mandy, Billy enters a werewolf in a dog show.
- Muppet Babies once had baby Animal mistake a FULLY GROWN DINOSAUR for a puppy.
- Truth in television, with sand cats often being mistaken for domestic cats.
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